The present invention relates to an ultrasound diagnostic apparatus and an ultrasound diagnostic method and particularly to an ultrasound diagnostic apparatus and an ultrasound diagnostic method for measuring the elastic characteristics of a vascular wall.
Conventionally, ultrasound diagnostic apparatus using ultrasound images are employed in the medical field. In general, this type of ultrasound diagnostic apparatus comprises an ultrasound probe having a built-in transducer array and an apparatus body connected to the ultrasound probe. The ultrasound probe transmits ultrasonic waves toward a subject, receives ultrasonic echoes from the subject, and the apparatus body electrically processes the reception signals to generate an ultrasound image and obtain various information on diseases.
The site of a subject to be ultrasonically examined varies with the kind of disease: in the case of a circulatory disease such as cerebral infarction, for example, carotid arteries are tentatively examined because plaques are liable to develop there. In such ultrasonic diagnosis of carotid arteries, the thickness of a plaque and the elastic characteristics of a vascular wall are measured to obtain information on, for example, the degree to which blood vessel stenosis has progressed and the ease with which the plaque may rupture. In order to obtain such information accurately, an ultrasonic beam is preferably allowed to enter a vascular wall at right angles thereto to accurately detect small changes occurring in the vascular wall as the heart beats. However, because the carotid arteries often do not lie parallel to the body surface in places where plaques tend to develop (where a common carotid artery divides into an internal carotid artery and en external carotid artery), allowing the ultrasonic beam to enter a vascular wall at right angles thereto was a difficult task.
JP 2001-299752 A, for example, describes an ultrasound diagnostic apparatus wherein an ultrasonic beam is steered to transmit the ultrasonic beam in various directions in order to select a direction in which a reflected ultrasonic echo exhibits the maximal amplitude as a direction in which the ultrasonic beam enters a vascular wall at right angles.
JP 2005-074146 A describes an ultrasound diagnostic apparatus wherein an ultrasonic beam is steered to pass through the center of a blood vessel so that the ultrasonic beam intersects the blood vessel at right angles.